Ductile-brittle Behavior Research Articles

Experimental characterization of Mode I fracture toughness of the undisturbed Guadalquivir Blue Marl: Effect of suction

Fracture Mechanics is an extensively developed discipline for a wide range of materials used in scientific and engineering applications. However, the thorough understanding and characterization of fracture phenomena in many soil and rock-type materials are still under development. This work aims to characterize the suction-dependent fracture response in undisturbed clay soils, which will lead to identifying a ductile–brittle behavior threshold. The undisturbed condition will provide information on the behavior of the soil in its natural state. For this purpose, a new experimental procedure is presented in order to obtain a closed-form relationship between mode I fracture toughness (KIc) and a suction range (Ψ) for the Guadalquivir blue marl. In order to achieve this objective, an experimental campaign was carried out using a series of semi-circular specimens subjected to the three-point bending test with different suctions. The corresponding loading–displacement curves from the tests are qualitatively analyzed to identify the fracture response of this material, and the linear regression technique is used to finally obtain the desired relationship. Current results show that an increasing value of suction increases both the maximum load and the stiffness, yielding a more brittle behavior associated with an increase in KIc. Finally, an alternative expression for the determination KIc with respect to that proposed by Kuruppu and coauthors (Kuruppu et al., 2013) is presented, which involves the explicit inclusion of the suction variable for the more brittle behavior, limited by the threshold found. This provides a new perspective on how moisture content affects the fracture mechanics of clays.

Evaluation of the Impact of Oxidation and Moisture Conditioning on Indirect Tensile Asphalt Cracking Test Parameters and the Tensile Strength Ratio

This study evaluated the effect of laboratory oxidation and testing temperature on asphalt mixtures’ moisture damage. The purpose of this study was to present a modified American Association of State Highway and Transportation Officials (AASHTO) T283 testing procedure that is more sensitive to moisture damage in asphalt mixtures. This study assessed the original test conditioning (OTC), short-term oxidative conditioning (STOC), and long-term oxidative conditioning (LTOC). Three performance grades (PGs)—PG 64-22, PG 76-22, and PG 52-34—and three test temperatures—the ambient laboratory temperature (25°C), asphalt intermediate temperature, and climate intermediate temperature (CIT)—were selected. To better understand the oxidation and moisture conditioning effects, AASHTO T283’s Tensile Strength Ratio ( TSR%) was utilized. Furthermore, additional analysis of the load–displacement curve parameters was carried out using the Indirect Tensile Asphalt Cracking Test, where an interaction diagram that included the mixture toughness ( Gf), ductile–brittle behavior ratio ( l75/ m75), and their interactions with CTindex was created. Statistical analysis was conducted using analysis of variance and Tukey’s post-hoc analysis to determine an appropriate oxidation and testing temperature where moisture damage is most sensitive. Results showed that higher oxidation and CIT had the greatest impact on TSR%. Moisture conditioning increased the CTindex and oxidation reduced the CTindex. The increase in CTindex was attributable to an increasing l75/| m75| and decreasing Gf. The CTindex decreased because of reducing l75/| m75| and increasing Gf. The statistical analyses showed that asphalt mixtures are more susceptible to moisture damage at STOC and LTOC than OTC. Overall, the findings highlight the need for an alternative oxidation conditioning and testing at the CIT to more accurately detect moisture damage in asphalt mixtures.

Fracture toughness of Fe–Si single crystals in mode I: Effect of loading rate on an edge crack (–110)[110] at macroscopic and atomistic level

This paper is devoted to an experimental and 3D atomistic study of the influence of loading rate on fracture toughness in dilute Fe–Si alloys and in bcc iron. We analyze new and previous experimental results from fracture tests performed at room temperature on bcc iron–silicon single crystals with edge cracks (1¯10)[110] (crack plane/crack front). The specimens of single edge notch-type were loaded in tension mode I under different loading rates. The ductile–brittle behavior at the crack front was monitored online via optical microscopy together with external force and prolongation of the specimens. About 30% decrease in fracture toughness was monitored in the new experiment under the highest loading rate. The nanoscopic processes produced by the crack itself were studied at room temperature via 3D molecular dynamics (MD) simulations in bcc iron under equivalent boundary conditions as in experiments to reveal (explain) the sensitivity of the crack to loading rate. For this purpose, this MD study utilizes the self-similar character of linear fracture mechanics. The results show that the emission of blunting dislocations from the crack is the most difficult under the highest loading rate, which leads to the reduced fracture toughness of the atomistic sample. This is in a qualitative agreement with the experimental (macro) results. Moreover, MD indicates that there may be some synergetic (resonant) effect between the loading rate and thermal activation that promotes dislocation emission.

Phase stability, mechanical, thermal, electronic properties, anisotropy, lattice dynamics and APB -energies of Ti2AlX intermetallics in α2, B2, and O phases: A first principle study

The ternary titanium aluminide based alloys have been lucrative for high-temperature applications in next-generation aerospace vehicles. Present work systematically investigates phase stability, structural and elastic properties of Ti2AlX (XMo, W, Ta, Nb, Zr, Hf) in α2, B2 and, O phases, calculated using density functional theory (DFT) within generalized gradient approximation (GGA). The Pugh’s ratio, Poisson’s ratio, Cauchy pressures, Vicker’s hardness, machinability index, anisotropies, Debye temperature, thermal conductivity, melting temperature, and Peierls stress, derived from the elastic constants are also presented. Phonon calculations reveal that Ti2AlX (XMo, W) in their α2 phase are dynamically unstable. The dominant slip systems are predicted from antiphase boundary (APB) energy calculations which indicate that dominance and mobility of the slip systems are largely affected by the ternary element ‘X′ and explains its ductile-brittle behavior. The bonding behavior is also investigated using the electronic density of states plots. The increased Ti-X bonding enhances while increased Ti-Al bonding degrades the ductility in Ti2AlX intermetallics.

Transition metal solutes in Ni-based ternary model superalloys: Partitioning and effects on elastic properties from first-principles calculations

Alloying effects on the structural, elastic, as well as electronic properties of Ni-based model superalloys are systematically investigated by first-principles calculations. Twenty-seven transition-metal(TM) elements (3d:Sc-Zn, 4d:Y-Cd, 5d:Hf-Au) are taken into account in Ni-based ternary model superalloys in detail for the first time. The partitioning behaviors and site preferences of ternary alloying elements are primarily predicted based on our calculated substitutional formation energies and partitioning coefficients, with previous atom probe tomography(APT) experimental results for comparison. Temperature dependence of partitioning behaviors of TM elements in model superalloys is also presented in this work. It is found that the partitioning behaviors of Cr, Mn, Tc, Ru and Os can be reversed at a specific temperature, and these elements just lie near the boundary between the two blocks of Ni-site preferences and Al-site preferences of TM elements on the periodic table. The bulk moduli, Young’s moduli, shear moduli, ductile–brittle behavior, orientation-dependent elastic moduli and elastic anisotropy of Ni-based ternary model superalloys are systematically estimated. The calculated elastic parameters agree well with the available experimental results. The effects of TM elements on elastic properties are found to be closely associated with their d-electron number. The elements at the beginnings and ends of the TM series reduce these elastic properties, while the TM elements lying towards the center of each period can considerably increase these properties of superalloys, especially those elements with a half-filled d band, which exert greater influences on improving the elastic performance. Furthermore, different charge redistributions caused by each alloying addition can elucidate different effects thereof on the elastic properties of model superalloys to some extent. The stronger directional bonding and more covalent-like interaction between TM additions and host atoms lead to the superior elastic properties.

Thermal Activation and Ductile vs. Brittle Behavior of Microcracks in 3D BCC Iron Crystals under Biaxial Loading by Atomistic Simulations

We present the results of free 3D molecular dynamics (MD) simulations, focused on the influence of temperature on the ductile-brittle behavior of a pre-existing central Griffith through microcrack (1¯10)[110] (crack plane/crack front) under biaxial loading σA and σB in tension mode I. At temperatures of 300 K and 600 K, the MD results provide new information on the threshold values of the stress intensity factor K and the energy release rate G, needed for the emission of <111>{112} blunting dislocations that support crack stability. A simple procedure for the evaluation of thermal activation from MD results is proposed in the paper. 3D atomistic results are compared with continuum predictions on thermal activation of the crack induced dislocation generation. At elevated temperature T and biaxiality ratios σB/σA ≤ 0.8 dislocation emission in MD is observed, supported by thermal activation energy of about ~30 kBT. With increasing temperature, the ductile-brittle transition moves to a higher biaxiality ratios in comparison with the situation at temperature of ~0 K. Near the transition, dislocation emission occurs at lower loadings than expected by continuum predictions. For the ratios σB/σA ≥ 1, the elevated temperature facilitates (surprisingly) the microcrack growth below Griffith level.

Prevention of Crack Formation in Electron-Beam Welded Joints of Dissimilar Metal Compounds (TiNi/Ti6Al4V)

The microstructure, mechanical properties and element distribution in an electron-beam welded joint of dissimilar metal compounds (TiNi/Ti6Al4V) have been studied. To prevent crack formation, niobium was introduced to the compound as a filler material. The welding line preheating and electron-beam deviation toward the Ti6Al4V alloy were used. It was shown that fracturing of the welded joint occurs within the fusion zone and is characterized by a mixed ductile-brittle behavior.

Dislocation emission and crack growth in 3D bcc iron crystals under biaxial loading by atomistic simulations

This paper is devoted to the study of the ductile-brittle behavior of a central nanocrack (1¯10)[110] (crack plane/crack front) under biaxial loading via free 3D molecular dynamics (MD) simulations, as well as the comparison of MD results with continuum predictions concerning T-stress. The so called T-stress is a constant stress component acting along the crack plane, which should be considered (together with the stress intensity factor K) in the assessment of brittle-ductile behavior, namely, in the case of the short cracks. Previous 2D atomistic simulations under plane strain conditions indicated that the level of T-stress (controlled by the biaxiality ratio σB/σA from the external loading) affects dislocation emission from the crack and can cause the ductile-brittle transition. The plane strain simulations using the periodic or translational boundary conditions in the bcc lattice have certain limitations: they enable the in-plane dislocation emission (Burgers vector lies in the observation plane), but they do not allow the complete dislocation emission on the all slip systems favored by the shear stress. As presented, our new free 3D atomistic simulations (without periodic or symmetry conditions) enable the activity of the all favored slip systems. Thus, they offer a more realistic insight into the microscopic processes generated by the crack itself in dependence on the T-stress level.

Theoretical Study of the Elastic and the Thermodynamic Properties of Re2C under High Pressure

The structural properties of Re2C in anti-MoS2 and anti-ReB2 structures have been investigated by using the pseudopotential plane wave methods based on the density functional theory. The anti-ReB2 structure is found to be more stable than the anti-MoS2 structure. In particular, for the first time, we have studied the elastic properties of Re2C in the anti-ReB2 structure under high pressure. The ductile-brittle behavior and Vickers hardness for the anti-ReB2 structure are also been analyzed. In addition, the Debye temperature, heat capacity and thermal expansion coefficient are discussed by using the quasiharmonic Debye model method.

Dynamic meshing force and blunt effects on atomistic scale ductile-brittle behavior of the crack in bcc iron

This work investigates the dynamic meshing force and blunt effects on atomistic scale ductile-brittle behavior of the crack in bcc iron through molecular dynamics. Two engineering factors including upload ratio and contact ratio are introduced to reveal the effects of force amplitude and distribution. Under different upload and contact ratios, normal stress fields are diverse and crack can propagate in ductile-to-brittle, weak ductile, and strong ductile ways. The increase of blunt layer leads to modest increase in critical applied load for crack ductile propagation, but it leads to obvious enhancement of crack toughness during single-tooth meshing.

Growth of 3D edge cracks in mode I and T-stress on the atomistic level

We present results of 3D molecular dynamic (MD) simulations accompanied by stress calculations on the atomistic level in 3D bcc iron crystals with edge cracks (1¯10)[110]. Two different crack lengths in samples of SEN type loaded uniaxially in mode I were treated with negative and positive values of T-stress according to the continuum prediction by Fett for constant stress boundary condition. Atomistic results are compared with continuum predictions, including influence of T-stress on the ductile-brittle behavior of cracks.

Ab initioprediction of the mechanical properties of alloys: The case of Ni/Mn-doped ferromagnetic Fe

First-principles alloy theory, formulated within the exact muffin-tin orbital method in combination with the coherent-potential approximation, is used to study the mechanical properties of ferromagnetic body-centered cubic (bcc) ${\mathrm{Fe}}_{1\ensuremath{-}x}{\mathrm{M}}_{x}$ alloys (M = Mn or Ni, $0\ensuremath{\le}x\ensuremath{\le}0.1$). We consider several physical parameters accessible from ab initio calculations and their combinations in various phenomenological models to compare the effect of Mn and Ni on the properties of Fe. Alloying is found to slightly alter the lattice parameters and produce noticeable influence on elastic moduli. Both Mn and Ni decrease the surface energy and the unstable stacking fault energy associated with the ${110}$ surface facet and the ${110}\ensuremath{\langle}111\ensuremath{\rangle}$ slip system, respectively. Nickel is found to produce larger effect on the planar fault energies than Mn. The semiempirical ductility criteria by Rice and Pugh consistently predict that Ni enhances the ductility of Fe but give contradictory results in the case of Mn doping. The origin of the discrepancy between the two criteria is discussed and an alternative measure of the ductile-brittle behavior based on the theoretical cleavage strength and single-crystal shear modulus $G{110}\ensuremath{\langle}111\ensuremath{\rangle}$ is proposed.

Theoretical study of electronic structure, phase transition, elastic, and thermodynamic properties of ReN

Phase transition of rhenium mononitride (ReN) in NaCl, CsCl, zincblende (ZB), NbO, wurtzite (WZ), NiAs, WC, PtS, Pmn21 and Cmc21 structures have been studied by using the projector augmented wave method. It is found that NbO-type structure is the most stable. This conclusion is consistent with the report of Wang et al., while contrary to the results of Zhao et al., Chen et al., Asvini et al., and Hlynsson et al. The phase transition from NbO-type to NiAs-type occurs at ca. 52.8GPa, which is also in good agreement with that of Wang et al. The elastic constants of NbO- and NiAs-type ReN under high pressure are calculated and found to be increased with the increasing pressures. At the same time, the ductile-brittle behavior is evaluated by Pugh's criteria. Also, we have predicted the density of states and Vickers hardness for NbO and NiAs types of ReN. Finally, the Debye temperature ΘD, thermal expansion α and heat capacity CV for NbO-type structure at high pressures are also derived through the quasi-harmonic Debye model.

Correlation between small punch and CVN impact tests for evaluation of cryogenic fracture characteristics of isothermally-aged nitrogen-containing austenitic stainless steels

The Charpy-V-notch impact test has been used routinely to evaluate the ductile-brittle behavior in steels. Likewise, the Small-Punch test has been applied to evaluate toughness properties in different materials. In this work, both tests were conducted at -196 °C on three types of austenitic stainless steels JN1, JJ1 and JK2, which were solution treated and then aged at temperatures between 700 and 900 °C for 10 and 1000 minutes. The solution treated steels exhibited a ductile fracture with high fracture energy, after testing with both tests. The brittle intergranular fracture was induced by the aging process of specimens and thus the fracture energy of tested specimens decreased dramatically. The highest and lowest decreases in energy with both tests occurred in the aged JN1 and JK2 steels, respectively. The Charpy-v-notch, CVN, test energy and small-punch, SP, test energy at -196 °C was found to follow a linear correlationship equation, CVN Test Energy = 89.7 SP Test Energy -63.0.

3D atomistic simulation of fatigue behavior of a ductile crack in bcc iron

We present results of 3D molecular dynamic (MD) simulations of the ductile–brittle behavior at an edge ( 1 ¯ 1 0 ) [1 1 0] crack in bcc iron loaded cyclically in mode I at temperature of 300 K. This crack in 3D bcc iron crystal under monotonic uniaxial tension loading in mode I emits dislocations in the inclined 〈1 1 1〉{1 1 2} slip systems and it is stable up to very high loads, in qualitative agreement with experiments and continuum predictions. Our question is how the cyclic loading changes the crack behavior and its stability.

Ductile-Brittle Behavior along Crack Front and T-Stress

We present new results of molecular dynamic (MD) simulations in 3D bcc iron crystals with embedded central through crack (001)[110] of Griffith type loaded in mode I. Two different sample geometries of the same crystallographic orientation were tested with negative and positive values of the T-stress, which change the ductile-brittle behavior along the crack front in 3D. This phenomenon is explained in the framework of stress analysis, both on the continuum and atomistic level.

3D atomistic simulation of the interaction between a ductile crack and a Cu nanoprecipitate

We present the results of molecular dynamic simulations of ductile–brittle behavior at an edge [1 1 0] crack neighboring with a Cu nanoprecipitate in bcc iron. This crack in pure bcc iron emits dislocations in the ⟨1 1 1⟩{1 1 2} slip systems and it is stable up to very high loads, in qualitative agreement with experiments and continuum predictions. Our question is how a copper precipitate influences the dislocation emission from the crack front and how the dislocations interact with the precipitate.

Ductile–brittle behavior at the (1 1 0)[0 0 1] crack in bcc iron crystals loaded in mode I

Fracture experiments performed at room temperature on four test samples made of Fe–3wt.%Si single crystals with an edge crack (1 1 0)[0 0 1] (crack plane, crack front) showed approximately 45° deflections of the crack from the initial crack plane (1 1 0). This behavior appeared to be independent on loading rate. Fractographic analysis confirmed that the cracks were deviated along {1 0 0} planes and the fracture was accompanied by dislocation slip and by twinning. 3D simulations at 300 K by molecular dynamic technique in bcc iron with edge cracks of equivalent orientation indicated that the crack itself could contribute to understanding of this behavior by three processes: twinning on oblique {1 1 2} planes, which hindered growth of the original crack, and by emission of dislocations on oblique {0 1 1} and {1 2 3} planes, which led to separation of the {1 0 0} planes and might cause decohesion and subsequent cleavage fracture along the mentioned planes.

Influence of crack orientation on the ductile–brittle behavior in Fe–3 wt.% Si single crystals

In this paper we present experimental results from fracture tests performed at room temperature on bcc iron-silicon single crystals with edge cracks of two different orientations (001)[110] and (− 110)[110]. The cracks were loaded under mode I. The fracture toughness and acoustic emission response were measured, and a fractographic analysis obtained via scanning electron microscopy was carried out. Experimental results confirm the basic predictions pertaining to the influence of crack orientation on crack stability from continuum modeling and molecular dynamic simulations in bcc iron.

Ductile-Brittle Behavior of Microcracks in 3D

Results of several parallel molecular dynamics crack simulations in bcc iron crystals with up to 128 million atoms are presented. The crack (001)[010] of Griffith type is loaded in Mode I. We observe dislocation emission and twinning near the free sample surfaces and later plastically induced crack initiation.